This invention relates to a power converter and, in particular, to a switching power supply such as a current resonance type DC/DC converter including a resonance circuit and a controlling method thereof.
In the manner which is well known in the art, the DC/DC converter is a power converter for converting an input DC voltage (which will later be merely also called an “input voltage”) into an output DC voltage (which will later be merely also called an “output voltage”) which is different from the input DC voltage.
As one of the DC/DC converters, there is a PWM (pulse width modulation) type DC/DC converter is known in the art. The PWM type DC/DC converters have various types which are classified into a step-down type, a step-up type, a polarity reversing type, or the like. The step-down PWM type DC/DC converter comprises an energizing switch, a short-circuit switch, and an output inductor. In lieu of the short-circuit switch, a diode may be used.
However, the PWM type DC/DC converter is disadvantageous in that it has a large switching loss when the energizing switch changes from an on state to an off state or changes from an off state to an on state. As a DC/DC converter which is capable of eliminating such a switching loss, a current resonance type DC/DC converter is known, for example, in U.S. Pat. No. 5,663,635 issued by Vinciarelli et al.
Although the current resonance type DC/DC converter will later be described in conjunction with FIGS. 1 and 8, the current resonance type DC/DC converter comprises a current resonance type DC/DC converting portion which includes an energizing switch, a resonance circuit, a short-circuit switch, and an output inductor. The energizing switch is turned on and off in response to a first driving control signal. The resonance circuit consists of a resonance inductor and a resonance capacitor. The resonance inductor has an end connected to the energizing switch. The resonance capacitor has an end connected to another end of the resonance inductor. The resonance capacitor has another end which is grounded. The short-circuit switch is connected in parallel with the resonance capacitor. The short-circuit switch is turned on and off in response to a second driving control signal. The output inductor has an end connected to the other end of the resonance inductor. The output inductor has another end connected to an end of an output capacitor. Each of the energizing switch an the short-circuit switch acts as a switching element.
In the current resonance type DC/DC converter, a current flows through the resonance inductor only for a resonance duration with respect to a switching period. The current does not flow through the resonance inductor for a duration obtained by removing the resonance duration from the switching period. When an input/output voltage ratio becomes smaller, the switching period with respect to the resonance duration becomes longer. As a result, durations where the current does not flow through the resonance inductor increase, as described, for example, in U.S. Pat. No. 4,720,667 issued by Lee et al.
The current resonance type DC/DC converter has a large advantage where a zero-current switching of the energizing switch is enable by using a series resonance of the resonance circuit consisting of the resonance inductor and the resonance capacitor, and it results in eliminating the switching loss.
In other words, generally, in the current resonance type DC/DC converter, the energizing switch is turned off on the moment at which a sum of the resonance current in the circuit and a load current becomes zero. That is, the reduction in the switching loss of the energizing switch is accomplished by performing the zero-current switching (ZCS).
The current resonance type DC/DC converting portion comprises a current detection arrangement and a voltage detection arrangement. The current detection arrangement is for detecting a current flowing through the energizing switch to produce a current detected signal. The current detection arrangement may be a current detection resistor inserted between the energizing switch and the resonance inductor. The voltage detection arrangement is for detecting a both-ends voltage of the resonance capacitor to produce a voltage detected signal. The voltage detection arrangement may be a signal line having an end connected to a connection node between the resonance inductor and the resonance capacitor.
The current resonance type DC/DC converter comprises a control circuit for producing the first and the second driving control signals. In other words, the control circuit is for controlling turning-on/off of the energizing switch and the short-circuit switch. Specifically, the control circuit comprises a first control portion for controlling the turning-on/off of the energizing switch and a second control portion for controlling the turning-on/off of the short-circuit switch.
The first control portion comprises a zero-current detection circuit connected to both ends of the current detection resistor, a first control logic circuit, and a first driver. The zero-current detection circuit detects whether a both-ends voltage of the current detection resistor is zero or not to produce a zero-current detected signal when the both-ends voltage of the current detection resistor is zero. The both-ends voltage of the current detection resistor corresponds the current detected signal. That is, the zero-current detection circuit is for detecting a zero-current on the basis of the current detected signal to produce the zero-current detected signal indicative of the zero-current. Responsive to the zero-current detected signal, the first control logic circuit produces an original high-side gate signal indicative of making the energizing switch turn off. Responsive to the original high-side gate signal, the first driver supplies a driving high-side gate signal to a control terminal of the energizing switch through a first signal line. The driving high-side gate signal corresponds to the first driving control signal.
On the other hand, the second control portion comprises a zero-voltage detection circuit connected to another end of the above-mentioned signal line, a second control logic circuit, and a second driver. The zero-voltage detection circuit detects whether a both-ends voltage of the resonance capacitor is zero or not to produce a zero-voltage detected signal when the both-ends voltage of resonance capacitor is zero. The both-ends voltage of the resonance capacitor corresponds to the voltage detected signal. That is, the zero-voltage detection circuit is for detecting a zero-voltage on the basis of the voltage detected signal to produce the zero-voltage detected signal indicative of the zero-voltage. Responsive to the zero-voltage detected signal, the second control logic circuit produces an original low-side gate signal indicative of making the short-circuit switch turn on. Responsive to the original low-side gate signal, the second driver supplies a driving low-side gate signal to a control terminal of the short-circuit switch through a second signal line. The driving high-side gate signal corresponds to the second driving control signal.
In the conventional current resonance type DC/DC converter, in a case of really equipping the control circuit, a timing at which the energizing switch is really turned off after the zero-current detection circuit detects the zero-current of the current flowing through the resonance inductor is delayed from an optimal timing caused by a first delay time of the first control logic circuit and a second delay time of an output of the first driver resulting from an inductance component of the signal line and a gate capacitance of the energizing switch. As a result, the conventional current resonance type DC/DC converter is disadvantageous in that the loss in the energizing switch is increased.
In addition, the above-mentioned first and second delay times in the control circuit change dependent on various conditions such as the load current and an ambient temperature. It is therefore very difficult to preliminarily grasp the delay amount in the control circuit quantitatively.
Similarly, the conventional current resonance type DC/DC converter controls to make the short-circuit switch turn on when the both-ends voltage of the resonance capacitor becomes zero volt. However, there is a delay until the short-circuit switch is really tuned on after detection of a zero-voltage in the both-ends voltage of the resonance capacitor. Accordingly, the conventional current resonance type DC/DC converter is disadvantageous in that the loss in the short-circuit switch is also increased.